Zero the irons first as I use them as my reference point when mounting optics, so knowing that they’re precisely on target is essential. Make sure you put a good number of rounds downrange to confirm the zero.
Now you can turn your optic on. With your iron sights up, adjust the optic so that the point of aim of your red dot (or whatever) matches up with the point of aim of your iron sights. Make sure to actually aim down your sights while doing this as if you were aiming at a target. Once that’s all settled fire a few rounds using only the dot to confirm the zero.
Congratulations! You now have a red dot with co-witnessed iron sights!